The invention relates to a tap changer for a transformer, especially cylindrical tap changers wherein an insulated cylinder is provided with fixed tap contacts for different voltage levels and a rotatable shaft provided with corresponding movable contact parts is arranged inside the cylinder. Each movable contact faces outwardly from the shaft towards the inside of the cylinder and comprises at least one or, for example, two mechanical contacts that can be brought into contact with the corresponding fixed contact of the cylinder.
In an example of a known type of tap changer that comprises a plurality of mechanical contacts that rotate together with the shaft, the mechanical contacts are arranged in parallel contact branches for connection to the fixed contacts of the tap levels. One of the mechanical contacts is connected in series with a vacuum interrupter switch, and the other of the mechanical contacts is connected in series with a second vacuum interrupter and a resistor. In a multi-phase system, the transformer comprises tap levels for each phase, and each phase includes two movable mechanical contacts arranged on the rotating shaft of the tap changer.
A tap changer of this cylindrical type is described in DE 10 2004 052 316. The document describes how the contact sequences of the tap changer are measured. The measurements are performed by connecting a measuring circuit between the two parallel branches. The measuring circuit is connected by removing the shaft from the cylinder, the measuring circuit is then connected to the two branches of the movable contacts via connecting cables, where-after the shaft is re-installed in the cylinder (§25). The measuring circuit is connected at a first connection point between the mechanical switch and the vacuum interrupter in the first branch, and a second connection point between the resistor and the vacuum interrupter of the second branch. The measuring circuit comprises a DC voltage source and a resistor in series and the voltage across the resistor is monitored (FIGS. 6 to 8). During a tap change, one of the branches is disconnected from the present tap level and is connected to the next level before the other branch. The measuring circuit registers voltage variations during the tap change, for example, from the opening of the vacuum interrupter of the resistor branch at the present tap level, to the closure of the vacuum interrupter of the other branch at the next level (§28). The measuring provides valuable information on the status of the switches, but is complicated and laborious.
Also, the reinstallation of the tap changer is complicated and may affect the tap changer and its contacts, so that the results of the measuring become invalid.
Moreover, this measuring circuit can not detect the opening of the mechanical switch in the resistor branch, when both branches are connected in parallel to the same tap.